The present invention relates generally to tracking system in rotary magnetic head type recording and/or reproducing apparatuses, and more particularly to a tracking system which can be applied to a magnetic recording and/or reproducing apparatus having a head moving mechanism for displacing a pair of rotary magnetic heads provided at mutually opposing positions, in mutually opposite directions along a plane which is perpendicular to the rotating plane of the two rotary magnetic heads.
A system wherein tracks are formed on a tape without gaps of guard bands between adjacent tracks has been previously developed. Moreover, a color video signal can be recorded and reproduced on such a tape without the occurrence of an interference beat disturbance. This system is described in U.S. patent application Ser. No. 731,935 (Now U.S. Pat. No. 4,178,606) entitled "Color video signal recording and/or reproducing system" filed Oct. 13, 1976, and assigned to the assignee of this application. In this previously developed system, a pair of azimuth heads have gaps which are inclined with a certain azimuth angle, in mutually opposite directions with respect to a direction perpendicular to the longitudinal direction which is of the track. Adjacent tracks are formed in contiguous contact side-by-side without a gap or guard band therebetween. The phase of the chrominance signal is shifted by 90 degrees for every horizontal scanning period. The direction of this phase shifting is reversed from one track to the next adjacent track. In accordance with this system, the tape utilization efficiency is high since the tracks are in close contact with each other. Moreover, there is no beat disturbance. PG,3
The prior art employs a system wherein a video signal is recorded on a magnetic tape in parallel tracks, formed obliquely to the longitudinal direction of the magnetic tape. This tape may either be stopped or transported at different speeds at the time of playback in order to carry out a speed-change in the reproduction, such as quick-motion reproduction, slow-motion reproduction, or still picture reproduction. In this known system, the tape travel speed at the time of reproduction is different from that at the time of recording. The tracing locus of the rotary head relative to the tape during reproduction differs from the tracing locus (track) of the rotary head during recording, whereby so-called tracking error occurs.
In the proposed system mentioned above, when tracking error occurs as a result of reproduction at changed speed, a so-called reverse tracking, in which one of the heads traces a part of a track recorded by a head having the same azimuth as the other head, occurs partially. With respect to this part traced by reverse tracking, there is almost no reproduction of the recorded signal because of azimuth loss. For this reason, when tracking error occurs, the reproduced signal level drops, and, in accordance with this, a noise component due to a reduction of the reproduced signal level accompanying tracking error will appear at irregular positions on the reproduced picture screen. For this reason, the noise part in the picture moves, and the S/N ratio of the entire picture becomes poor. Consequently, a speed-change reproduced picture cannot be obtained in good picture quality.
Accordingly, as a system for correcting for tracking error at the time of changed-speed reproduction, a control system in which a head moving mechanism is provided respectively for a pair of rotary magnetic heads on a rotary structure, and the magnetic heads are caused to undergo displacement by the head moving mechanism, whereby each rotary head accurately carries out correct tracking, has been proposed.
On the other hand, recently in helical scan type magnetic recording and/or reproducing apparatuses for home use (hereinafter referred to as VTRs) having rotary magnetic heads, high-density recording and reproduction is being developed due to improvements in the magnetic tape and increased density in the rotary magnetic heads. Accordingly, VTRs have been realized which are capable of performing a long duration recording or reproduction such as a six-hour recording or reproduction by use of a conventional magnetic tape for performing two hours of recording or reproduction, for example, by reducing the tape speed, track pitch, and the like, to one-third that of the conventional system. However, since the tape driving system in the VTR for home use is simplified in order to reduce the cost, it is difficult to stably maintain the desired tracking accuracy by following and scanning along the curves in the video track, due to the increased density in the recording and reproduction. Especially upon a so-called interchanged reproduction in which a magnetic tape recorded by a certain VTR is reproduced by another VTR, it is even more difficult to stably maintain the above tracking accuracy, and a desirable picture quality could not be obtained.
Therefore, a precise tracking control of the magnetic heads is required so that the magnetic heads trace exactly on the tracks, especially for high density recording and/or reproduction. The head moving mechanism is also effectively applicable to meet with the above described requirement. In this head moving mechanism, the rotary magnetic heads are displaced in a plane which is perpendicular to the rotating plane of the magnetic heads, in a direction perpendicular to the longitudinal direction of the track. As the above described head moving mechanism, there are types which use piezoelectric elements respectively provided at a plurality of rotary magnetic heads, and types which perform a so-called see-saw operation like the head moving mechanism proposed in a U.S. patent application Ser. No. 178,852 entitled "MAGNETIC RECORDING AND REPRODUCING APPARATUS WITH DEVICE FOR TRACKING CONTROL OF ROTARY MAGNETIC HEADS" filed Aug. 18, 1980, issued Dec. 21, 1982 as U.S. Pat. No. 4,365,279, in which the assignee is the same as that of the present application.
In the system which uses piezoelectric elements in each of the plurality of rotary magnetic heads, the rotary magnetic heads are controlled so as to accurately perform positive tracking, by separately displacing each of the rotary magnetic heads. However, in this control system, it is necessary to provide a piezoelectric element in each of the rotary magnetic heads, and the construction of the system accordingly becomes complex. Furthermore, in a case where inconsistencies exist in the operations of the piezoelectric elements, this system was disadvantageous in that not all of the rotary magnetic heads can be finely controlled to perform accurate positive tracking.
On the other hand, in the proposed system using the see-saw operation, when one rotary magnetic head is moved by a predetermined amount in a predetermined direction, the other rotary magnetic head moves in a direction opposite to the above predetermined direction by an amount equal to the above predetermined amount. Thus, under the same condition in which the diameter of the drum is identical, the movable range of the rotary magnetic heads can be made larger than that in the above system using the piezoelectric elements. Accordingly, even when the variable speed ratio is large, a reproduced picture can be obtained having no noise bar upon special reproduction modes. However, since a permanent magnet part and a yoke part are required in order to perform the see-saw operation, the weight of the head moving mechanism increases. Hence, even when a driving current respective of the tracking error is provided by use of a closed loop servo loop to perform the tracking control, the responses of the rotary magnetic heads become slow in following and tracing the tracks. Moreover, the waveform of the above driving current was complex, and difficult to form. Furthermore, in order to obtain a 1/N (N is an integer) speed slow-motion reproduction picture, by scanning a track on a magnetic tape which is continuously moved at a tape speed equal to 1/N times the tape speed upon recording, a part where the differentiated value is infinity is required in the driving current waveform. That is, a part which shows a finite change although the time is zero, is required in the driving current waveform. In addition, this system was disadvantageous in that the system was sensitive to phase deviation, and thus, stable tracking operation could not be obtained.
On the other hand, a method of performing a slow-motion reproduction in which no noise bar is introduced, has also been proposed for a helical scan type VTR having no head moving mechanism and using an azimuth recording and/or reproducing system requiring no guard bands. In this proposed method, it was perceived that there were positions at which still picture reproduction can be performed without introducing noise, even though the rotary magnetic heads crosses the border between the tracks. Therefore, by this proposed method, reproduction is performed in the so-called noiseless still picture reproducing position, and the magnetic tape is moved so that the movement of the magnetic tape is in synchronism with the rotation of the rotary magnetic heads, and so that the rotary heads do not cross the border between the tracks. Next, reproduction is performed at another noiseless still picture reproducing position by use of a control signal, and the above operation is repeatedly performed. Accordingly, by varying the time ratio at which the above described operation is repeated, the desired slow-motion ratio can be obtained. According to this proposed method, the cost of the system is low since no head moving mechanism is required, and the system can be suitably applied to a VTR for home use.
However, in this proposed method, some instability of the picture is introduced, since the magnetic tape is moved intermittently. Moreover, some undesirable effects are introduced, due to the necessity to use rotary magnetic heads having substantially large track widths compared to the width of the recording track.